1. Field of the Invention
The present invention relates to a cam follower with roller, and more particularly, to a cam follower with roller that is incorporated into a valve mechanism of an internal combustion engine of a vehicle.
2. Description of the Background Art
A combustion engine having a variable valve timing mechanism such as a VTEC (Variable Valve Timing & Lift Electronic Control System) is generally known.
In such a combustion engine, a valve mechanism is configured such that two driving locker arms and a connecting locker arm connecting them actuate together when operated at a high speed, whereas the two driving locker arms actuate separately when operated at a low speed.
In each of the driving locker arms and the connecting locker arm, a roller contacting a cam is supported by a needle bearing. Currently, as a material for a shaft supporting the roller, SUJ2 is used in the driving locker arms, whereas carburized steel is used in the connecting locker arm. These materials are used for the following reasons.
At low engine revolutions, a switching pin does not actuate, generating a small load applied on the needle bearing supporting the roller. Thus, an SUJ2 material is used for the driving locker arms.
At high engine revolutions, however, the switching pin actuates so that the shafts of the driving locker arms and the connecting locker arm are connected, generating an excessive load applied on the needle bearing supporting the roller in the connecting locker arm. The bearing must therefore have a large width in order to ensure long life.
Moreover, when the number of revolutions is high, the driving locker arms and the connecting locker arm actuate together by the actuation of the switching pin, which may cause a problem of shaft bending that originates from the side of the inner diameter of the shaft. This is addressed by increasing the width of the bearing and by using a tough carburized material for the supporting shaft of the roller in the connecting locker arm.
It is noted that the SUJ2 material used here is quenched and tempered such that the hardness of a rolling surface of the needle roller is adjusted to HRC 60 to 62.
The roller supported by the locker arms is in direct contact with the cam. In this portion, lubrication easily dies away compared to other engine components, and thus the contact surface of the cam and roller is called a boundary lubrication region. The needle roller supporting the roller is in contact with the shaft basically in a manner of rolling contact. Under the severe circumstances as described above, however, small sliding occurs between the needle roller and the shaft when the cam load varies rapidly. This makes the needle roller bearing have poor lubrication, causing abrasion and the like, resulting in insufficient durability. Furthermore, as the cam load increases, the pressure on the contacting surface of the needle roller and the shaft also increases, shortening fatigue life.